4.10 Autotropic reactions

The hyphae of many fungi can alter their direction of growth to avoid growing
into each other and accentuate exploration of uncolonised regions of the
substratum. The avoidance mechanism or negative autotropism
(Fig. 16) is particularly evident at low hyphal densities, in regions such as
the margin of the growing colony. The ability of hyphae to sense the presence of
another hypha is thought to be due either to a localised depletion of oxygen
around the target hypha, a higher concentration of carbon dioxide, or the
presence of a secreted metabolite.

Fig 19. Autotropisms: diagrammatic
illustrations of negative autotropism leading to the hyphal avoidance
reaction, which is so crucial to the colonial growth form of filamentous
fungi. The drawings show the responses of pairs of hyphae of Aspergillus
nidulans, Mucor hiemalis and a wild type strain of
Neurospora crassa; the approaching hypha is shown in blue in each case.
The point where the avoiding reaction was first observed is indicated by the
arrow. The numerals give times in minutes. The medium was overlaid with
cellophane membranes prior to inoculation to keep the hyphae on the surface
and prevent hyphae diving beneath the surface. Modified and redrawn from
Trinci et al., 1979.

The minimum distance of approach of two hyphae before negative autotropism
caused one to grow away was 30, 27 and 24 μm respectively in Neurospora
crassa, Aspergillus nidulans and Mucor hiemalis (Trinci
et al., 1979; see Fig. 19).

In a maturing mycelium, autotropism can be reversed so that young hyphae are
attracted (possibly chemotropically?) to an older hypha (called positive
autotropism). This can lead to hyphal fusions. In one mechanism the
target hypha is induced to branch, and the consequential tip to tip contact is
followed by breakdown of the two apices and fusion between the two hyphae (Fig.
20). For more detail see
Section 5.17.

Fig. 20. Autotropisms: sketch showing positive
autotropism leading to a ‘tip-to-peg’ hyphal fusion reaction. This is one of
the fusion reactions leading to the interconnected mycelial network of the
mature colony as illustrated in Fig. 4.6. Other reactions are tip-to-tip and
peg-to-peg (see Fig. 2 in Chapter 7). For more detail see Fig. 13 in Chapter
5 and refer to Hickey et al., 2002 and Glass et al., 2004.
For more detail see
Section 5.17.

This process converts the central regions of a maturing colony into a fully
interconnected network through which materials and signals can be communicated
efficiently and this enables the vegetative mycelium to make best use of its
resources. In the formation of fruit bodies and similar structures, positive
autotropism enables many hyphal tips to congregate together to initiate the
developing tissue. In these cases hyphal fusions and adhesions may be rare
(though they can be used to bind the structure together); instead, developmental
regulation organises the concerted contribution of many independent hyphal tips
to formation of the tissues and structures of the fruit body (discussed in
Chapter 9 &
Chapter 12).